OBSOLETE

LMC2001

SNOS011D – AUGUST 1999 – REVISED APRIL 2013

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APPLICATION INFORMATION

The Benefits of LMC2001 No 1/f Noise

Using patented methods, the LMC2001 eliminates the 1/f noise present in other amplifiers. This noise which

increases as frequency decreases is a major source of measurement error in all DC coupled measurements.

Low frequency noise appears as a constantly changing signal in series with any measurement being made. As a

result, even when the measurement is made rapidly, this constantly changing noise signal will corrupt the result.

The value of this noise signal can be surprisingly large. For example: If a conventional amplifier has a high

frequency noise level of 10nV/

√Hz and a noise corner of 10 Hz, the RMS noise at 0.001 Hz is 1µV/ √Hz

This is equivalent to a 6µV peak-to-peak error. In a circuit with a gain of 1000, this produces a 6mV peak-to-peak

output error. This number of 0.001 Hz might appear unreasonably low but when a data acquisition system is

operating for 17 minutes it has been on long enough to include this error. In this same time, the LMC2001 will

only have a 0.51mV output error. This is more than 13.3 times less error.

Keep in mind that this 1/f error gets even larger at lower frequencies.

At the extreme, many people try to reduce this error by integrating or taking several samples of the same signal.

This is also doomed to failure because the 1/f nature of this noise means that taking longer samples just moves

the measurement into lower frequencies where the noise level is even higher.

The LMC2001 eliminates this source of error. The noise level is constant with frequency so that reducing the

bandwidth reduces the errors caused by noise.

Another source of error that is rarely mentioned is the error voltages caused by the inadvertent thermocouples

created when the common “Kovar type” package lead materials are soldered to a copper printed circuit board.

These steel based leadframe materials can produce over 35uV/°C when soldered onto a copper trace. This can

result in thermocouple noise that is equal to the LMC2001 noise when there is a temperature difference of only

0.0014°C between the lead and the board!

For this reason, the leadframe of the LMC2001 is made of copper. This results in equal and opposite junctions

which cancel this effect. The extremely small size of the SOT-23 package results in the leads being very close

together. This further reduces the probability of temperature differences and hence decreases thermal noise.

Overload Recovery

The LMC2001 recovers from input overload much faster than most chopper stabilized opamps. Recovery, from

driving the amplifier to 2X the full scale output, only requires about 50ms. Most chopper stabilized amplifiers will

take from 250ms to several seconds to recover from this same overload. This is because large capacitors are

used to store the unadjusted offset voltage.

The wide bandwidth of the LMC2001 enhances performance when it is used as an amplifier to drive loads that

inject transients back into the output. A to Ds and multiplexers are examples of this type of load. To simulate this

type of load, a pulse generator producing a 1V peak square wave was connected to the output through a 10pF

capacitor. (Figure 26) The typical time for the output to recover to 1% of the applied pulse is 80ns. To recover to

0.1% requires 860ns. This rapid recovery is due to the wide bandwidth of the output stage and large total GBW.

Figure 26.

No External Capacitors Required

The LMC2001 does not need external capacitors. This eliminates the problems caused by capacitor leakage and

dielectric absorption, which can cause delays of several seconds from turn-on until the amplifier is settled.

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